Loaded transmission circuit



July 22 1924.

T. SHAW LOADED TRANSMISSION CIRCUIT 'Filed June 30, 1922 loaded (idleark 2 52 INVENTOR.

Z Jiwaw c WTTORNEY Patented July 22, 1024.

warren are THOMAS SHAW, OF HACKENSACK, NEW JERSEY, ASSIGNOR TO AMERICANTELEPHQN I AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

LOADED TRANSMISSION CIRCUIT.

Application filed June30, 1922. Serial No. 571,849.

To all whom it may concern:

Be it known that I, THOMAS SHAW, residing at Hackensack, in the countyof Bergen and State of New Jersey, have invented certain Improvements inLoaded Transmission Circuits, of which the following is a specification.

Thisinvention relates to the loading of transmission circuits, and moreparticularly to the loading of transmission circuits which are to beemployed for the transmission of carrier frequencies.

In multiplex systems employing carrier frequencies for transmissionpurposes, it has been found that open-wire lines are best adapted fortransmission of carrier frequencies, and, in general, cable circuits ofconsiderable length cannot be employed for this purpose, because thecapacity and resistance of the cable tends to greatly increase theattenuation of the higher frequencies. In the practical layout of a planemploying carrier transmission, however, the use of some cable lengthsin a given circuit cannot well be avoided by' reason of the fact thatcable is used in entering practically all large cities and is used forterminal purposes.

The resistance, inductance, capacity and conductance respectively of acable, are.inherently different from those of open-wire line. Inconsequence of these differences, the presence of cable in an open-wireline circuit causesimpedance irregu arities at t-he junction points ofthe cable; and the open-wire line. These irregularities are transmittedto the telephone repeaters and cause reduction in the gain of thetelephone, repeaters. These irregularities also cause re'fiexion lossesand transmission distortion.

The impedance irre ularity may be approximately eliminate by loading theinserted cable so thatit has as nearly as possible the samecharacteristic impedance as the open-wire line with which it isassociated. In doing this the loading coil inductance and the spacing ofthe coils should be so chosen in accordance with the present inventionthat the loaded cable circuits will transmit efficiently the differentcarrier frequencies which are transmitted over the open-wire line.

As is well known, it is common practice to obtain three telephonecircuits from each group of four wires by using each of two pa rs ofwires as a side circuit and by using the two pairs together as a phantomcircuit.

In the application of carrier transmission to telephone circuits of thistype, however, it is not always expedient to use phantom circuits forcarrier tele hone and telegraph transmission systems, ecause ofcross-talk difliculties which would be encountered. Carrier systems areusually only superposed on where, as is usually the case, a'lp antomgroup includes cable in the open-wire circuit, it is desirable .to loadthe side circuits to a cut-ofl frequency 'sufliciently high to transmitthe carrier frequencies ordinarily used, while loadin the phantom withthe usual lower cut-o frequency loading employed in connection withordinary voice transmission. 1

The expedient of loading the phantom to the same cut-ofi frequency asthe side circuits is not in general practicable because the number ofloading coils per unit length increases with the cut-off frequency, andto load the phantom with the additional coils necessary "to bring it upto the same cut-off frequency as the side circuits would be extremelyexpensivewhen it is considered that in the usual case no additionaltransmission channels would be obtained from the phantom by the extraloading because the phantom is unavailable for carrier transmission"tion to provide a system of loadin whereby the side circuits may be usedtransmission while the phantom circuit will be loaded only for ordinaryvoicev transmission.

Carrier systems as now operated require efiicient transmission offrequencies. up to approximately 30,000 cycles. In order to obtainsatisfactory transmission and im edance characteristics in the entrancean in termediate cables through'which carrier systems'are routed, it isnecessary to space the or carrier wire lines, very satisfactory resultsare obtained by spacing loading coils at intervals of about 5600 feet.

In view of the fundamentally different transmission requirements ofcarrier circuits superposed on the sides of phantoms and theordlnaryspeech channel over the phantom circuit itself, it is convenientand economical to load each transmission circuit system with independentsets of coils spaced at approximately the intervals abovementioned. Tnpractice, it has been found that six loading coils inserted in thecarrier side circuit for each loading coil inserted in the voicefrequency phantom satisfies the transmission requirements above referredto and at the same time conforms to the exigencies of the ordinary plantpractice. 7

The use of two difierent sets of coils for the carrier circuit and thephantom circuit loading involves certain diiiiculties in that thephantom circuitloading coils add certain effects to the carrier circuitswhich acteristics in the carrier circuits.

should be properly allowed for in the design of the loading, in order toobtain satisfactory transmission and impedance charloading coils I occurmuch less frequently than the carrier circuit loading coils L Tn thedrawing, 8, and 8, represent the side circuit drops at theterminalofiice, and S and S designate the corresponding openwire lineconnections terminating at some oint outside of the ofice; In thecircuits 1 and S phantom terminating repeating transformers A, and A,are inserted, and taps are taken from the midpoints of the cells ofthese transformers to forni the phantom drop P llntervening between theterminal ofiice and the terminals of the open-wire circuits 8, and S,are a number of loading points numbered from 1 to 6 inclusive. Theseloading points are preferably equally spaced, the spacin between twoadjacent loading points cing designated on the drawing as S. Side cir-'cuit coils L, are included in each side circuit at each of the loadingpoints indicated, and in addition phantom coils L are included at theloading points numbered 2 and 5. Thus it will beseen that, asilluscoils.

Corre actose trated in the circuit shown, there is one phantom coil forevery three side circuit This number has been chosen in or der tosimplify the illustration in the drawing, but it will be understood thatin actual practice, where the carrier range extends to a frequency inthe neighborhood of 30,000 cycles, a greater number of side coils willoccur for each phantom. Six side circuit coils to each phantom circuitcoil have been found to be a satisfactory number, as has already beenstated.

Considering first the reactions of phantom loading upon the carriercircuit loading installed on the side circuits, it will be obvious thatowing to the fact that the phantom loading coils occur much lessfrequently than the side circuit loading coils, the resistance,inductance and capacity efl'ects which the phantom loading coils add tothe side circuit tend to cause impedance irregularities. The conductanceefi'ects of the phantom coils upon the side circuit are negligible.

The coil resistance effects may be minimized by designing the coils soas to obtain as low a resistance as is economically practicable inmeeting the other electrical requirements. The inductance added to thecarrier circuit by the phantom coils due to magnetic leakage in thecoils is, however, a somewhat more serious problem. This maybecompensated for by making special adjustments in the carrier circuitloading coil, which is installed at the phantom loading point. In thedesign of the phantom coil, as will be pointed out later, the capacityeli'ects of the coil are of primary importance, and hence the leakageinductance of the phantom coil must, to a large extent, be permitted toattain whatever value is determined by capacity considerations. Thecarrier circuit loading coil installed at the phantom loading point isdesigned to have an inductance smaller than the inductance of the otherside circuit coils by an amount equal to the inductance of the phantomcoil in the side circuit. In this connection it should be noted thatgeneral considerations of economy and plant flexibility makes itdesirable to install the phantom loading coil at the carrier loadingpoints, so that the inductance can be taken care of in the mannerstated.

As an example, suppose the loading of the side circuit requires a sidecircuit coil at each'loading point having an inductance of 5.25milhenrys. Suppose, on the other hand, that the inductance of thephantom coil in the side circuit at the phantom loading point adds .10milhenrys to the side circuit. itis necessary to bring the side circuitinductance to within one per cent 10f the figure given above for a sidecircuit coil. it will be apparent that the inductance added by thephantom involves a greater variation in this section than will bepermissible. Accordingly, the inductance in this section may be takencare of, so far as a side circuit is concerned, by providing a coil forthe side circuit at this loading pointhaving-an inductance of 5.15milhenrys, so

' that the total loading inductance in the side trical balance. Thisbalance is'necessary in order to avoid objectionable phantom-tosidecross-talk. This .construction causes an appreciable direct capacitybetween the two windings. The windings also have a direct capacityto thecase in which the coils are potted and a direct capacity to the loadingcoil core, which consists of iron. The distribution of these capacities,which aifect the side circuit transmission, is fully discussed in U. S.Patent to Campbell and Shaw, Se-

rial No. 980,921, of January 10, 1911, the capacity distribution beingshown in the diagram of Fig. 3 of said patent.

It will be obvious that the capacities above referred to produce aconsiderable capacity between the conductors of the side circuit at theloading point. These capacity efiects of the phantom loading coil uponthe side circuit may be allowed forby treating the phantom coil as theequivalent (so far as capacity is concerned) of a building-out condenserinthe side circuit, which is used for carrier purposes. As is wellknown, it is now the practice, where a section is shorter than thenormal loading section, to shunt a capacity across the terminals of thesection at the loading point, the capacity being designed to increasethe capacity of the short section to that of a normal section. Thispractice has also been followed in the loading of circuits for carrierpurposes. In fact, owing to the short spacing of the carrier circuitloading coil and the irregular spacing of the cable manholes in whichthe loading coil pots are normally installed, it is more frequentlynecessary to install building-out condensers in carrier loading sectionsthan in ordinary loading sections. Where it is necessary to supplybuilding-out condensers in order to make the capacity of irregularcarrier loading sections equal to the theoretical value of the loadingdesign, it is not a severe hardship to allow also for the presence ofcapacity due to phantom loading coils as building-out condensers in thecarrier circuit.

In new cable installations, however, in volving carrier loadinginitially or ultimately, it is desirable and. usually will'bepracticable to lay out the cable manholes to fit the theoretical carrierloading spacing, and it should not be necessary in a plant thus designedto use building-out condensers to equalize the loading section capacity.Under such conditions, the capacity introduced into the carrier circuitsby the phantom loading coils constitutes a source of objectionableirregularity. In long cables, the irregularity efi'ects pile up at.certain important carrier frequencies because of the uniform periodicityof the recurrence of the individual irregularities. For example, inpractical loading systems involving six side circuit loading sections toeach phantom loading section, and where carrier loading sections areapproximately 900 feet long, the phantom coil irregularity efi'ects,piles up at frequencies'which are approximate multiples of 11,000cycles.

Under the conditions above specified, the effect of phantom coilcapacity in the side circuit may be taken care of in either of two ways:(a) by selecting for the carrier loadmg sections which contain thephantom loading coils cable circuits which have proportionately lowercapacities than the regular carrier loading sections which do notcontain phantom loading coils; (b) an equivalent reduction in capacityin the car rier' loading section which contains the phantom loading coilmay be obtained by geographically shortening this leading section.

Considering now the effects of the carrier circuit loading in the sidecircuit, upon the phantom loading, it will be seen that the the sidecircuit is more or less masked by the comparatively large capacity ofthe phantom independently of loading. This is due to two reasons: First,the ordinary speech frequency phantom loading sections haveapproximately ten times as much mutual capacity as the ordinary carrierloading section's, this being in part due to the fact that an ordlnarytype of phantom circuit has a mutual capacity per unit length which isapproximately sixty per cent greater than that of its own side circuits,and in part due to the approximate 6:1 ratio between the physicallengths of the phantom and carrier loading sections; second,'the coilmutual capacity effective in the phantom circuit is in,- herentl muchless than the coil capacity which is effective in the side circuits.Thislatter relation will be obvious when we consider that the principalcapacity introduced by the side circuit loading coils is between the twoconductors of the side circuit, and substantially no capacity isinvolved between the windings of the coil in one side circuit and thewindings of the coil in another side circuit, for the two coils areentirely separate structures and will be located physically aconsiderable distance apart, as compared with the distance between thetwo windings of a given side circuit coil. Even the capacity introducedby the phantom loading coil in the phantom itself may be disregarded, asin constructing the phantom coil, the windings in one side of thephantom circuit are wound upon different quadrants of the core from thewindings in the other side of the phantom, and the capacity between thewindings of the two sides of the phantom will be quite small, due to thephysical separation of the windings. Therefore, the capacity introducedinto the phantom by the phantom loading coil itself is not ordinarilyconsidered in the design of the loading coil or of the loading.

Since the capacity may be "disregarded as above stated, it will beapparent that the principal effect of the carrier or side circuitloading coils in the phantom circuit is due to their resistance andleakage inductances. As the carrier or side circuit loading coils areusually of the air-core type (because of high frequencies employed incarrier trans mission), the resistance and inductance effects-arerelatively larger than those which occur in ordinary loading, where theiron core type of loading coil is employed. The relatively largeresistance of the air-core coils is a potential source of impedanceirregularity at low speech frequencies, where the resistance of theconductor and the coils is an important factor in determining thecharacteristic impedance of the loaded cable. If the ratio of resistanceto inductance per unit length in the loaded cable is different from thecorresponding ratio of the associated open wire line, there will be adifference in impedance which will cause irregularity at the low speechfrequencies transmitted over the phantom circuit. This effect will ofcourse also occur at the low frequencies transmitted over the speechfrequency channels of the carrier circuits.

In order to prevent the impedance irregularities due to thisditferenceinratio from becoming objectionably large, it is desirable to design thecarrier loading coils to definite resistance requirements,"whieh dependupon the gauge of the cable conductor adopted as standard. No definiterule can be laid down, but in general the best practicable value ofresistance is determined by a study in which cost considerations arebalanced against transmission reactions.

menace Coming now to the inductance efi'ects upon the phantom circuit ofthe carrier loading coils employed in the side circuit, it will be foundthat these inductance eflects are greater than in the case of theiron-core coils which are used in the ordinary side circuit loading forordinary voice frequencies. This is due in part to the employment of theair-core type of coil, and in part to the design of the winding of thecoil. The aircore, being of very low permeability, producescomparatively large magnetic leakage, and this magnetic leakage producesa considerable inductance in the phantom, whereas, if the coil wereperfect magnetically, the side circuit coil would produce no inductancein the phantom, owing to the balance-of the windings. In designing thewindings of the coil, it is necessary to employ a construction whichwill obtain a low mutual capacity. This necessitates the use of arelatively large amount of insulation between the inner and outersection windings of the coil, with the consequence that there is agreater separation between the windings. The increased separation, whilereducing the mutual capacity, increases the magnetic leakage and resultsin a further increase in the inductance in the phantom, due to the sidecircuit coil.

Tn carrier loading coils of the type above described, the leakageinductance effect just considered amounts to nearly four per centwhereas with the ordinary type of loading coil, the leakage whichproduces inductance in the phantom from the side circuit coils is muchless than one per cent. This increased inductance effect in the phantomcircuit may be readily cared for where the side circuits are loaded forcarrier transmission, by considering the leakage inductance effects inthe phantom as distributed inductance. This is entirely proper where thephantom is only used for the transmission of ordinary voice frequenciesbecause of the much closer spacing of the carrier side circuit loadingcoils. Tn other words, the phantom loading eifect of the side circuitcoils is electrically equivalent to continuous loading and can be addeddirectly to the distributed inductance of the phantom cable circuit indesigning the phantom coil load ing.

' As a practical example, it may be stated that the effects of carrierloading coils in an existing commercial circuit employing the principlesof the present invention supply approximately fifteen per cent of thetotal inductance per unitlength, which is necessary for satisfactorytransmission over the cable phantom circuits. Therefore, the inductanceof the phantom loading coils required on the phantom circuits of thisinstallation, in which the side circuits are loaded for carrieroperation, is approxilZO mately eighty-five per cent of the inductanceof the coils which would be required if the side circuits were loaded inthe ordinary way for ordinary voice frequency transmission. The phantomloading effect of the carrier circuit loading coils thus makes necessarythe use of different inductances in the phantom loading coils than arerequired in the; ordinary voice loading of toll entrance cablesconnected with nonloaded lines.

It will be obvious that the general principles herein disclosed may 'beembodied in many other organizations widely different from thoseillustrated, without departing from the spirit of the invention asdefined in the following claims.

lVhat is claimed is:

1. A transmission system including a phantomed group of conductors, thephantom circuit being loaded to transmit one limited range offrequencies and the side circuits being loaded to transmit anotherlimited range of frequencies. one of said ranges being considerablywider than the other.

2. A transmission system including a isfactorytransmission only withinthe voice range and the side circuits being loaded with coils designedfor high frequency transmission and spaced at proper intervals to givesatisfactory transmission for carrier currents above the voice range.

.5. A transmission system including a phantomed group of conductors, thephantom circuit being loaded by coils spaced at certainsubstantially'uniform intervals and the side circuits being loaded bycoils spaced at quite differentbut substantially uniform intervals.

6. A transmission system including a 1 phantomed group of conductors,the phantom circuit being loaded by coils spaced at certainsubstantially uniform intervals and the side circuits being loaded bycoils spaced at more frequent but substantially uniform intervals.

7. A transmission system including a phantomed group of conductors, theside circuits being loaded by coils spaced at certain intervals and thephantom circuit being loaded by coils spaced at intervals correspondingto multiples of the side circuit intervals.

8. A transmission system including a phantomed group of conductors, theside circuits being loaded by coils spaced at certain intervals and thephantom circuit being loaded by coils spaced at intervals correspondingto multiples of the side circuit intervals, said intervals being sorelated that each phantom loading point will correspond to a sidecircuit loading oint.

9. A transmission system inc uding a phantomed group of conductors, thephan- I tom circuit being loaded by coils designed andspaced at suchintervals as to give satisfactory transmission over a range offrequencies, the side circuits being loaded with coils designed for highfrequency transmission and spaced at such intervals as to givesatisfactory transmission for a range of frequencies higher than thattransmitted by the phantom circuit, the spacing of the coils being sochosen that the'phantom loading points correspond to sidecircuit loadingpoints, building-out condensers being includedinjthe side circuits tobuild out those sections which are electrically shorter than thestandard side circuit section and the capacity introduced into the sidecircuit b the phantom loading coil at phantom loa ing oints constitutingat least part of a buil ing-out condenser for its section.

10. A. transmission system including a phantomed group of conductors,loading coils in the side circuits spaced at certain interv vals,loading coils inserted in the phantom circuit at points corresponding toside circuit loading points and at intervals constituting multiples ofthe side circuit intervals,

' building-out condensers included in the side circuit sections whichare shorter than the standardloading section and the capacity of thephantom loading coil in the side circuit constituting at least apart ofa buildingout condenser for the corresponding side circuit section.

11. A transmission system including a phantomed group of conductors,loading coils in the side circuits spaced at certain intervals, loadingcoils inserted in the phantom circuit at points corresponding to sidevcircuit loading points and at intervals constituting multiples of theside circuit intervals, the side circuit section including the phantomloading point being electrically shorter than the standard side circuitsection and the capacity of the phantom loading coil in the side circuitconstituting a condenser to build out the short section.

12. A transmission system including a phantomed group of conductors.loading coils included in the phantom circuit at certain intervals andloading coils included in the side circuits at more frequent intervals,the loading coils in the phantom circuit being so designed as toincrease the inductance of the phantom to transmit a desired range offrequencies, the leakage inductance of the carrier circuit coils uponthe phantom" increasing the distributed inductance of the phantomcircuit, and the inductance of the loading coils for the phantom loadingpoints being proportioned to compensate for such increase.

13. A. transmission system including a phantomed group of conductors,loading coils included in the side circuits at intervals sufiicientlyfrequent to enable the side circuits to transmit frequencies above thevoice range, loading coils included in the phantom circuit at intervalsconstituting multiples of the side circuit intervals to enable thephantom to transmit voice frequencies, the inductance added by thephantom coils being proportioned to give the proper transmissioncharacteristic upon the assumption that to the normal distributednsoneac inductance of the phantom the leakage inductance in the phantomdue to the side circuit loading coils is added as additional uniformlydistributed inductance.

14-. A} transmission system including a phantomed group of cableconductors connected to a corresponding phantom group of open-Wireconductors, loading coils included in the phantom circuit at suchintervals as to load the phantom for voice transmission and loadingcoils included in the side circuits at such intervals as to load theside circuits for the transmission of carrier frequencies above thevoice range, the Winding resistance of the side circuit coils and thephantom loading coils being so proportioned with respect to cableconductors of a given gauge as to give the loaded cable circuitsapproximately the same ratio of resistance to inductance per unit lengthas that obtaining in the open-Wire line. i

lln testimony whereof, I have signed my name to this specification this29th day of June, 1922.

THUMAS SHAW.

